1. Field of the Invention
The present invention relates to an image forming apparatus such as printers, copying machines, and facsimile machines. More specifically, the present invention is directed to an image forming apparatus equipped with an intermediate transfer member.
2. Description of the Related Art
In an image forming apparatus using an electro-photographic system such as printers, copying machines, and facsimile machines, image forming operations are carried out as follows: That is, after electrostatic latent images are acquired by electrostatic latent image forming unit in such a manner that light is irradiated onto image carriers such as photosensitive drums which have been uniformly charged, toners are applied to the charged electrostatic latent images by developing unit so as to produce visible images. Then, a plurality of toner images are transferred to intermediate transfer members in a multiple mode, and thereafter, these multiple toner images are transferred onto recording mediums so as to be fixed thereon.
As an example of conventional image forming apparatus, a rotary type developing unit disclosed in JP-A-8-328348 will be described hereinbelow. While a rotary type developing unit is provided in which four sets of color developing devices (yellow, magenta, cyan, black developing devices) functioning as developing unit are held in the vicinity of a photosensitive drum along a circumferential direction of a rotary member, toner images which are sequentially formed by the apparatus are transferred onto a transfer belt in an overlapping manner (refer to pages 3-4 and FIG. 1 of JP-A-8-328348).
In an image forming apparatus employing such an image forming system as disclosed in JP-A-8-328348, while a full-color image is outputted, a yellow toner image, a magenta toner image, a cyan toner image, and a black toner image are transferred (primary transferred) onto a transfer belt every time the transfer belt is rotated by one turn, so that these four color toner images are sequentially overlapped with each other on the transfer belt. From the portion where all of these four color toner images have be overlapped with each other, a toner image is transferred (secondary transferred) onto a recording at a secondary transfer position by employing, for example, a secondary transfer roller. At this time, if the secondary transfer roller is depressed to the transfer belt before all of these four color toner images are overlapped with each other, then intermediate toner images being overlapped are scratched up by the secondary transfer roller. To avoid the problem, the following method has been employed. That is, while the secondary transfer roller is separated (retracted) from the transfer belt until all of the four color toner images are overlapped with each other, the secondary transfer roller is push out (advanced) toward the transfer belt at required timing in order that the secondary transfer roller may be made in contact with the transfer belt.
In the image forming apparatus, after a secondary transfer operation of a toner image is accomplished at the secondary transfer position, residual toners are present on the transfer belt. These residual toners are such toners that could not be transferred to a recording medium and thus are left on the transfer belt. As a consequence, generally speaking, in the image forming apparatus, cleaners used for intermediate transfer members are installed around the transfer belts in order to remove the above-described residual toners (remaining toners). These cleaners for the intermediate transfer members are arranged under such a condition that these cleaners can be contacted/released (advanced/retracted) with respect to the transfer belts. For example, the cleaner used for an intermediate transfer member is configured as follows. That is, while the cleaner is retracted (separated) from a transfer belt in order not to scrape a toner image before being transferred (secondary transferred) until a toner image has been transferred (primary transferred) onto a transfer belt, the cleaner is caused to abut against the transfer belt at predetermined timing, so that toners left on the transfer belt after the secondary transfer operation has been accomplished may be scraped.
As explained above, in the conventional image forming apparatus, the image forming operations are carried out by employing such a construction that both the secondary transfer rollers and the cleaners for the intermediate transfer members are contacted/released (advanced/retracted) with respect to the transfer belts. The contacting/releasing operations of the secondary transfer rollers and the cleaners for the intermediate transfer members may cause shocks as well as vibrations with respect to image forming units. For example, while transfer rollers normally employ rollers made of rubber, these rubber transfer rollers are depressed against transfer belts under considerably large weights. As a result, the contacting/releasing operations of the transfer rollers may constitute load variations. For example, if shocks occurred when the transfer rollers abut against the transfer belts are transferred to the image forming units, then image disturbances may occur, which may cause color shifts. Also, load variations given to intermediate transfer belts may cause, for example, rotation fluctuations of photosensitive drums. Further, in such a case that both a photosensitive drum and a transfer belt are combined as an integral unit, load variations given to an intermediate transfer belt may vibrate the integral unit itself, which may sometimes cause color shifts during light exposing operation.
In this case, for example, as described in the above-explained patent publication JP-A-8-328348, when the distance between the primary transfer position and the secondary transfer position in the transfer belt can be made long, then the image forming apparatus may be designed by such a design concept that the secondary transfer roller may abut against the transfer belt after, for example, the last color toner image has been transferred onto the transfer belt, or after, for instance, the exposing operation used to form the final color toner image has been accomplished. However, in the case that a length of a transfer belt is made longer and a processing speed of an image forming process is increased, either the transfer belt or a photosensitive drum are necessarily required to be rotated in high speeds. To this end, high power is necessarily required, so that output power of a motor corresponding to a drive source is increased, and thus, power consumption is similarly increased. On the other hand, generally speaking, when lengths of transfer belts are made long, the image forming apparatus become bulky, so that the apparatus become unable to satisfy current requirements as to compactness/light weights of such image forming apparatus.